Polyester-type thermoplastic elastomer (below “TPE”) has good oil resistance and chemical resistance as well as excellent low temperature impact property. Thus, it has been widely used in automobile and electric/electronic fields. However, particularly chloroprene rubber (CR), which has been widely used for automobile parts, has poor durability. Thus, recently CR has been replaced with TPE. The use of TPE has been expanded in North America and Europe since it has a good light weight property, resistant-fatigability, chemical resistance, and ozone resistance, compared with CR. Particularly, since TPE for extrusion blowing is produced through blowing procedure in the melting state, resin should have good melt viscosity and melt tension in the melting state, and the thickness distribution of processed product during extrusion blowing should be constant.
Generally speaking, TPE cannot be subjected to extrusion-blowing due to its low melt viscosity and melt tension in the melting state. The melt viscosity or melt tension of TPE may be increased at melt polymerization by using branching agent, but is not sufficient. In order to solve this problem, there was an attempt to increase the melt viscosity and melt tension of TPE by using crosslinking agent in the extruder after melt polymerization, but constant physical properties of TPE could not be obtained. For instance, according to U.S. Pat. No. 4,071,503, hydroxyl group and carboxylic acid group, the terminal groups of elastomer, are reacted with using diisocyanate or polycarbodiimide to improve melt viscosity and melt tension, and so could obtain TPE which may be subjected to extrusion blowing. However, its productivity was poor since the residence time in the extruder was unduly long. U.S. Pat. No. 5,733,986 discloses that the number of terminal groups of TPE is controlled in order to make hydroxyl group and carboxylic acid group, the terminal groups of TPE, fully react, thereby inducing the reaction of isocyanate group with hydroxyl group and the reaction of carboxylic acid with polyepoxy compound, to obtain TPE for blowing. However, the method did not solve the problems of thermal resistance and residence time yet. Diisocyanate group shows rapid reactivity with hydroxyl group, and polyethoxy compound shows rapid reactivity with carboxylic acid, thereby inducing the increase of viscosity. However, a sufficient residence time in the extruder is needed to control the difference of reaction rate between the above groups, which is closely related to productivity. Particularly, diisocyanate mostly reacts with hydroxyl group in the chain, and thus if the residence time in the excluder is insufficient, the reaction is not occurred enough. As a result therefrom, gas by unreacted diisocyanate at extrusion blowing may be generated. Also, unreacted diisocyanate may induce a continuous reaction in the remelting process at polymer processing, which also affects the control of polymer processing condition. Therefore, the reaction of diisocyanate should be fully done, and the variation in physical property by product lot should be minimized by increasing reactivity.